In recent years, a fuel cell has been attracting increasing attention as a highly efficient energy conversion apparatus. Fuel cells can be broadly divided by the type of electrolyte used, into low temperature operating fuel cells such as an alkaline-type, a solid polymer-type, a phosphoric acid-type, etc., and high temperature operating fuel cells such as a molten carbonate-type, solid oxide type, etc. Among them, a polymer electrolyte fuel cell (PEFC) which uses a polymer electrolyte membrane having ionic conductivity as an electrolyte attracts attention as a stationary power source, vehicle mounted power source, and mobile phone power source since it has a compact structure and provides high power output density and does not use a liquid electrolyte and can be operated at low temperature so that it can be realized as a simple system.
A solid polymer electrolyte fuel cell has gas diffusive electrode layers disposed on both surfaces of an electrolyte membrane with an anode-side exposed to fuel gas (hydrogen gas or the like) and a cathode-side exposed to oxidant gas (air or the like), and is based on the basic principle that water is produced by chemical reaction via the polymer electrolyte membrane such that the reaction energy produced in this reaction can be extracted as electric energy.
When hydrogen and oxygen are supplied to an anode and cathode, respectively, as an active material, reaction (1) takes place on the anode catalyst and reaction (2) takes place on the cathode catalyst and electric power is generated by the electric potential difference.H2→2H++2e− (E0=0 V)  (1)O2+4H++4e−→2H2O (E0=1.23 V)  (2)
Since a fuel cell system has highly efficient power generation characteristics, it is now being developed for practical application, and durability for practical application has been achieved in stationary operations. However, since platinum is used as the electrode catalyst, the system is expensive, and it has also become evident that, in non-stationary operations such as during fuel depletion or startup/shutdown operation, irreversible deterioration of the system is accelerated. These problems need to be overcome for practical application of a fuel cell.
Mechanism of anode deterioration caused by depletion of fuel (hydrogen gas or the like) supplied during operation of a fuel cell will be described below. When the fuel is depleted, one or both of the reaction (3) in which water is decomposed in electrolysis to produce H+, and the reaction (4) in which carbon that carries an anode catalyst reacts in corrosion to produce H+, takes place in order to supplement H+ that is required for cell reaction, and causes considerable deterioration of the anode.H2O→½O2+2H++2e−  (3)½C+H2O→½CO2+2H++2e−  (4)In particular, deterioration of anode due to reaction (4) is serious and may lead to instantaneous breakage of the fuel cell. Especially when reaction efficiency of electrolysis of water is poor (that is, reaction overvoltage is high), the reaction (4) in which H+ is produced by corrosion of carbon as the catalyst carrier is more likely to take place than the water electrolysis reaction (3), leading to large deterioration of anode.
Mechanism of cathode corrosion due to startup/shutdown operation will be described below. In a stationary operation of a fuel cell, there is a hydrogen atmosphere on the anode side and air atmosphere on the cathode side. In startup/shutdown operation, in general, air is supplied to the anode side to stop generation of electricity. In a state of operation stop, usually there is air surrounding both the anode side and the cathode side, and in order to start generation of electricity (startup), hydrogen gas is supplied to the anode in the air atmosphere. When hydrogen gas is supplied to the anode at the time of startup, a mixture of hydrogen and air may possibly be present on the anode side.H2→2H++2e−  (1)O2+4H++4e−→2H2O  (2)½C+H2O→½CO2+2H++2e−  (4)Thus, at the time of startup of operation, on the portion of the anode near the inlet port for anode gas where hydrogen gas is being supplied, the hydrogen oxidation reaction (1) takes place, and on the portion of the opposite cathode in position opposed to the anode gas inlet port where air (oxygen) is already present, oxygen reduction reaction (2) takes place, so that, on the upstream portion of the anode and the cathode, a reaction system of ordinary fuel cell takes place. On the other hand, on the portion of the anode near the anode gas outlet port where air (oxygen) supplied during the operation stop remains and hydrogen is not yet supplied, oxygen reduction reaction (2) takes place. On the portion of the opposite cathode in position opposed to the anode gas outlet port, corresponding oxidation reaction takes place. However, since no hydrogen is present to be oxidized, corrosion reaction (4) takes place to oxidize the carbon. Thus, on the portion of the cathode opposed to the downstream portion of the anode, a reaction system of carbon corrosion takes place, and this is reported to be one of the causes of cathode deterioration during startup/shutdown operation (Patent Literature 1).
In Patent literature 2, as a means for preventing deterioration during the fuel depletion, a method is disclosed in which a water electrolysis catalyst such as iridium oxide is mixed with the electrode catalyst in order to prevent the anode catalyst carrier from being corroded during hydrogen depletion. In accordance with this method, a fuel cell can be made more tolerant to cell reversal.
In Patent Literature 3, in a fuel electrode of a solid polymer electrolyte type fuel cell, a method relating to a fuel electrode (anode) of solid electrolyte fuel cell is disclosed which comprises at least one reaction layer accelerating the reaction of the fuel cell in contact with the solid polymer electrolyte membrane and at least one water decomposition layer in contact with the diffusion layer for electrolysis of water within the fuel electrode (anode). It is reported that, in accordance with this method, a solid polymer electrolyte type fuel cell can be provided in which deterioration of electrode is unlikely to be produced even when lack of fuel occurs in the fuel electrode.
Also, as a means for preventing deterioration during the startup/shutdown operation, adoption of highly crystallized carbon as a carrier of the cathode catalyst is exemplified, and an example of platinum black usage have been disclosed (Patent Literatures 4, 5).
Impurities such as CH4, C2H6, C2H4C6H6CO2, CO, and the like may be contained in the fuel gas. Among these impurities contained in the fuel gas, CO is known to be specifically adsorbed to Pt, and cause degradation of performance. This makes it difficult to reduce consumption of the anode catalyst.